Method of developing high physical properties in ferrous material and product produced thereby



Jan 18, 1966 M w FT 3,230,118

METHOD OF DEVELOPING HIGH PHYSICAL PROPERTIES IN FERROUS MATERIAL AND PRODUCT PRODUCED THEREBY Filed Aug. 15, 1961 I NVENTOR.

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United States Patent METHOD OF DEVELOPING HIGH PHYSICAL PROPERTIES IN FERROUS MATERIAL AND PRODUCT PRODUCED THEREBY Marshall W. Tufts, Crete, Ill., assignor, by mesne assignments, to Screw and Bolt Corporation of America, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 15, 1961, Ser. No. 131,557 7 Claims. (Cl. 148-12) My invention relates to ferrous materials having improved physical properties and to a method of improving the physical properties of ferrous materials. It is specifically directed to a method of improving the physical properties of ferrous materials that respond to strainhardening or precipitation-hardening of some nature while maintaining the hardness at least substantially constant, or, alternately, while reducing the hardness.

It is well known that the physical properties of many ferrous materials, such as carbon steels, can be improved by heat treatment. To the best of my knowledge an increase in physical properties by heat treatment is always accompanied by an increase in hardness. In applications in which hardness is the limiting factor, such as applications in which machinability is important, the known methods of improving the physical properties are of little benefit.

My invention, in its broadest aspect, consists essentially of stressing, at least once, ferrous materials that respond to strain-hardening or precipitation-hardening of some nature to at least a point at which, upon release of the stress, a permanent set or plastic deformation is imparted to the steel. The stressing operation may optionally be performed in conjunction with a cold working operation, generally one in which a change in the cross section configuration of the material is achieved, such, for example, as a cold drawing operation. If a cold working operation is employed, it may precede or follow the stressing operation.

Alternately, the ferrous material may be stress relieved in conjunction with the stressing, or stressing and cold working operations. The stress relieving should be carried out at a temperature below the recrystallization temperature of the ferrous material in its unstressed condition. The stressing, cold working if employed, and stress relieving need not occur in any particular sequence although the advantages of the invention will not be achieved if the ferrous material is first stress relieved and then stressed obviously. It will often prove to be more expedient to first stress and then stress relieve, and this is the preferred method. It is quite within the scope of my invention however to simultaneously stress and stress relieve the material in one operation.

Further, more than one stress relieving operation may be employed. If a plurality of stress relieving operations are employed, they may be used after each preceding stressing and/or cold working operation, or after only selected preceding operations.

My invention will best be understood from the following description which, when read in light of the accompanying drawings, will illustrate several methods by which it may be carried out. In the accompanying drawings,

FIGURE 1 is a diagrammatic representation of the range of plastic deformation on representative stress-strain curves;

FIGURE 2 is a stress-strain curve of a C-lOlS type steel; and

FIGURE 3 is a stress-strain curve of a 4140 type steel.

For purposes of convenience my invention will be described in terms of treatment of bar stock. It should be understood that the scope of the invention is not limited to bar stock. The physical properties of ferrous material 3,230,118 Patented Jan. 18, 1966 of almost any type and cross-sectional configuration, regular or irregular, can be improved if the material can somehow be subjected to suflicient stresses.

For purposes of this description and claims the terms permanent set and plastic deformation refer to a condition of the material in which it has been stressed to a point such that, upon release of the stress, a permanent set or plastic deformation will have been imparted to the material. The magnitude of plastic deformation need not be equivalent to the deformation during stress. The material will be considered to have a permanent set if it does not return to its original condition upon release of the stress.

In materials in which the yield joint is readily discernible, the lower end of the range of plastic deformation may be equated roughly to it, but as is well known the yield point is not readily discernible in many types of conventional materials, such as cold drawn bar stock.

The upper end of the range is defined by that point on a stress-strain curve beyond which rapid loss of strength is produced by continued application of stress. When speaking of bar stock, for example, the upper end of the plastic deformation range would be that point at which the material begins to neck down.

FIGURE 1 illustrates diagrammatically the plastic deformation range on two typical steels.

Curve A represents the stress-strain curve of 4140 hot rolled carbon steel bar stock having the following approximate analysis:

C .40 Mn .86 P .015 S .025 Si .25 Cr .93 Mo .20

Curve B represents a steel which has been hot rolled, cold drawn and stress relieved prior to being treated in accordance with my method.

In the steel of curve A the lower end of the plastic deformation point would be approximately at point C, which is roughly the yield point of the steel. The upper end of the plastic deformation range would be represented by point D, which is the breaking point of the steel. The plastic deformation range would therefore extend from C to I) along the abscissa.

The yield point of the steel represented by curve B is not readily discernible. If the steel were stressed in the range represented, roughly, between E and D, upon removal of the stress the steel would return at least partially to its original condition. Since the yield point is not readily discernible, the lower end of the plastic deformation range cannot be pinpointed on the curve but can be described as the point beyond which the steel will not return to its original condition. The upper end of the range would be represented by point F, since that part of the curve between F and G represents the necking-down of the steel before it fractures at G.

My invention can best be visualized from data derived from tests run on conventional types of bar stock.

Table I shows to what extent the physical properties of 4140 bar stock was improved with a constant stress relieving temperature. The particular stock tested had the following approximate analysis:

In each instance specimens approximately 3 feet long x inch in diameter were first stressed to a point in the plastic deformation range and thereafter stress relieved. Following these two steps the indicated tests were run to evaluate the physical properties.

the fact that the physical properties, for example the yield strength, increased appreciably. The last column indicates the hardness normally required to reach the physical properties achieved by the combination stressing and stress relieving operations specified in column 1.

Table I Normal Hard- Hardness T.S. Y.S. EL. 2" R.A. ness, Equivalent H. to Achieved Physicals Q.&T. 1,180 F 143, 800 136, 800 19. 4 57. 5 279 Q.&T. 133, 800 20. 56. 271 294 Q,.&T. 149, 400 17. 7 57. 5 271 301 Q.&T. 152, 900 16. 6 54. 3 286 327 Q.&T 168, 500 16. 6 47. 9 286 353 In its quenched and tempered condition, the steel had a yield strength of 136,800 p.s.i. Whereas after it was stressed 8 percent and then stress relieved at 600 degrees F. the yield strength was increased to 168,500 p.s.i., an increase of approximately 26 percent in terms of the original yield In Table II the improvement in physical properties achieved by subjecting specimens of 4140 bar stock to gradually increasing stress relieving temperatures while maintaining the stress approximately constant are tabulated.

strength. In the foregoing table and throughout the specification the magnitude of stress is expressed in terms of increase in length over the original length of the specimen. The increase in length will of course be accompanied by a reduction in cross sectional area. Unless otherwise indicated, all materials having a definitely discernible yield point were stressed beyond it. The length of time the specimen was subjected to the stress relieving temperature was based on conventional industry practice. Usually the bars were stress relieved approximately one hour per cross-sectional inch, with approximately a 50 percent safety factor.

The figures show that the combination of stressing followed by stress relieving increased the strength of the steel over its quenched and tempered condition. Temperatures of 200 F. to 800 F. were effective, with maximum improvement occurring at about 400 F. Again a comparison of the values in the last two columns indicates that the increase in physical properties was achieved with practically no increase in hardness.

In Table III the increase in physical properties to be expected when my method is used in conjunction with a conventional cold drawing and stress relieving operation is illustrated.

The last two columns should be compared with the strength columns. The hardness, expressed in terms of Brinell hardness numbers, BHN, an untreated specimen in its quenched and tempered condition was 279. In the remainder of the specimens the hardness remained constant within experimental error.

In this table the first 3' specimen of hot rolled 4140 bar stock was subjected to a cold drawing operation which reduced its diameter and then stress relieved at 600 F. The second specimen, which was identical in chemical composition to the first, was first stressed 6 This occurred despite percent and stress relieved at a temperature of 400 degrees F. before the cold drawing and final stress relieving operation. The additional steps produced more than a 21 percent increase in tensile strength and almost a 21 percent increase in yield strength. In this instance, the hardess f the specimen dropped markedly. This is important and then stress relieved at 400 degrees F. From a comparison of the curve, specimen M showed markedly higher strength properties while the hardness was maintained substantially equal to the specimen represented by curve L.

because it makes possible the use of material for a given A typical method of treating ferrous material in accordapplication whose initial hardness is too high and strength ance with my invention is outlined below. too low for the application. Steel bar stock in either a hot rolled, normalized, In Table IV the effect of varying both the amount of quenched and tempered, or annealed condition will be stress and the stress relieving temperature is indicated. 10 used as an exemplary material. My method is applicable A C1444 type steel having the following composition was to bar stock in all conditions. tested: C .42, Mn 1.57, P .016, S .290. The steel bar stock, in its condition as delivered from Table IV i Normal EL. 2 Hard- Hardness T.S. Y.S. percent RA ness, Equivalent B.H.N. to Achieved Physicals The second test piece was first stressed 6 percent, then the mill, is first subjected to tensile stress in the plastic stress relieved at 400 degrees F. and finally cold drawn. deformation range. By comparison, the same procedure was followed with Either concurrently with or after the stressing operathe third test piece with the addition of a final stress tion the steel is stressed relieved. The stress relieving relieve at 600 degrees F. The additional stress relieving temperature and the time at which the steel is subjected operation markedly improved the tensile and yield to the temperature will vary on many factors including, strengths. The increase in physical properties in speciamong others, the size of the workpiece, the analysis of men number 3 was achieved with substantially no inthe steel, the grain structure, and the amount of final crease in hardness over the reference specimen or the residual stresses desired. With hot rolled round stress relieved and cold drawn specimen. type C1018 bar stock I have subjected 3' specimens to The foregoing tabular data can be visually summarized temperatures in the range of from about 200 degrees F. from the typical stress-strain curves of FIGURES 2 and 3. to 800 degrees F. for approximately an hour.

In FIGURE 2 a C1018 type steel having the following The type of heat employed is not especially important. approximate analysis is illustrated: If a bar is stress relieved simultaneously with the stressing, electric 'heat can be used. Gas heating could just as well C 13 be utilized however. Mn After stressing and stress relieving, the steel bar stock P 012 may be subjected to a further sizing operation. I have S applied my method particularly to cold drawing operations. The cold drawing steps indicated in the figures curve H lfipfesents a hot rohd, Cold drawn speclmelh and preceding tables represent cold drawing operations curve I represents a hot rolled, 6 Percent Stressed: 400 in which the reduction in diameter of the bar stock was degrees F. stress relieved, cold drawn, and 600 degrees F. approximately 1/ 1 i h to k l h th t stress I'BllCVBd specimen. Specimen K W215 first StISSd the cold drawing peration may be used merely as a 6 percent in its hot rolled condition, stress relieved at i i gperafign 40( degrees F. and cold drawn. The stressing and stress Th c ld d i Operation i t advantageously Tehfivlng operatihhs Preceding the Cold drawing p in carried out at room temperature and I define the term each of the p h h J and K materially improved the cold drawing as used in this specification and claims to h Characterlstlcshardness of the Various describe a drawing operation in which the dies are exposed speflmefls Stayed constant Wlthlll the limit of P to ambient temperatures. The dies may, of course, pick mentalermrup heat during the drawing operation but the addition In FIGURE 3 p Steel havlng the following of heat is not necessary to the practice of my invention approxlmate analysis 15 Illustrated: and, in fact, I prefer that the dies be unheated.

The bar stock after stressing, stress relieving and cold C .40 Mn 84 6O drawlng may then be used 1n 1ts cold drawn condition. P .015 The stock will, however, contain stresses, particularly 3 surface stresses as a result of the drawing operation. In Si 25 order to minimize, and perhaps eliminate these stresses, Cr .93 a second optional, stress relieving operation can be per- Mo 20 formed. The manner of performing the second stress relieving operation can be identical to the first stress In this figure, as in FIGURE 2, the improved physical relieving operation. Temperatures in the range of from properties resulting from my method are compared to about 200 degrees F. to 1000 degrees F. have been cmthe physical properties achieved by a specimen which has ployed to advantage. been hot rolled, cold drawn and stress relieved at 600 7 Alternately, the improvement in physical properties degrees F., the hot rolling, cold drawing and stress remay be achieved by the use of ultrasonics in conjunction lieving operations being carried out in a conventional with the stressing operation. Briefly, a workpiece is submanner. The hot rolled, cold drawn, and stress relieved jected to ultrasonic sound waves as it is stressed. To specimen is represented by curve L. The hot rolled specithe best of my knowledge, the method of applying the men represented by curve M was first stressed 6 percent ultrasonic sound is not critical. I have utilized both the transmission method, in which a crystal is placed at both ends of a bar to provide a through emission of sound waves, and the reflection method, in which one crystal is placed adjacent the bar and the waves bounce off the crystal and back to the oscillator. The results of both clean as if it had been subjected to a conventional scale removal operation.

The improvement in tensile and yield strength properties in ferrous materials will vary widely with the type of material, and particularly the carbon. In some types methods of ultrasonics in conjunction with the application of steel the improvement may be less marked than in those of stress are tabulated in Table V. All specimens were discussed herein. It may still be extremely desirable, hot rolled round 4140 stock. however, to employ my method on such a steel because Table V Normal EL. 2 RA. Hard- Hardness T.S Y.S. percent percent ness, Equivalent Re to Achieved Physicals BAR #1 6% 152, 000 47.3 30/32 33 HR 6% 400 161,800 7 41.2 31/32 36 HR 6% 400 168,100 7 41. 2 34/35 37. 5 HR 6% C.D., 650 172,500 3.5 43.8 3s

BAR #2 HR 6% 163,500 163,000 5.5 41.2 29/30 345 HR 6% 400 176,100 171,100 5. 5 42.8 34/35 33 The transmission method was used with specimens the machinability may be increased. For example, in 2 through 4 and the reflection method with specimens many applications, such as sucker rod connectors, ma- 5 and 6. In both methods a frequency of 2 /2 megachinability is the limiting service factor of the steel. A cycles was used during the time the specimens were subsucker rod connector is essentially a sleeve type coupling jected to stress. In specimens 3 and 6 the hardness inwhich connects two adjacent lengths of pipe used in oil creased slightly over the hardness of specimen 1. The well drilling. Generally, the connector is machined in amount of hardness increase was, however, considerably its heat-treated condition because heat treating a maless than the hardness that would have been required to chined piece would tend to distort the threads. In order reach the achieved physicals. to be machinable the steel is limited to about 32 Rock- Experimentation has also indicated that the length well C which in turn limits the inherent strength of the of time that a stress is held on the specimen is not es- 35 stock. By utilizing my method it is possible to obtain pecially important. The following table shows the physimproved machinability with steels having a higher ical properties produced in chemically identical specistrength than a limit of 32 Rockwell C would dictate. mens subjected to a holding stress of varying duration. My method will also be useful in treating structural Each specimen was a 3 foot piece of 4140, i round steels. It is desirable to increase the physical properties bar stock. Each was first stressed 6 percent, and the of structural steels while reducing the mass for obvious stress maintained for the times set forth in the table. reasons. At the present time it is generally not possible Table VI to increase the physical properties of structural steels by heat treatment because most heat treating furnaces can- H M, T S EL 2 R A H (in not handle structural steel sizes. Even if they could, the i fg heating and quenching operations required would create distortion, and it is extremely difficult to compensate for 0 154,100 30 distortion in structural shapes. Since my method imparts 30 144,200 10.0 50.0 28/29 60 144,200 m 0 m 0 28/30 little or no d1stort1on 1t 1s poss1ble to achieve the benefits 120 142,800 10. 0 53. 0 28/29. 5 of heat treatment w1thout the adverse s1de effects.

It should also be noted that in some cases my method As can be seen, the achieved physical properties are W111 actually reduce the hardncss i the Steel trasted to the hardness of the starting stock wh1le 1mw1th1n the range of expenmental error from specimen to provmg the phys1cal propertles. specimen. Th1s is an important economic cons1derat1on To the best of my knowledge, the type of stress 1ms1nce a commercial operation need not be slowed down parted to the ferrous material 1s not especially cr1t1cal. to prov1de stress dwell times.

Generally, I contemplate that it Wlll be more expedlent to I have also observed that the ach1eved physical propert th ties do not vary significantly with differences in the rate lmpart a tensl.e S ress a.co.mpreSSwe The foregomg descr1pt1on 1s intended to be 1llustrat1ve of appl1cat1on of the stress. Th1s factor 1s l1kew1se an only as other applications of my 1nvent1on Wlll at once be important commercial cons1derat1on because production apparent to those skilled 1n the art. Those applicatlons of bar stock in large quantitles need not be slowed due to a limifin rate of a lication 0f Stress are, of course, w1th1n the scope of my 1nvent1on. It 1s g pp my intention that the scope of my invention be limited One great advantage of my method 1s that the convenonly by the scope of the followmg clalms. tional scale removal step generally employed by bar stock I claim fabricators can be eliminated. Hot rolled or annealed 1 I thod of im amino increased h Sisal r0 bar stock as it comes from the mill has gone through the n 6 3 b p t 1 k y g f last phase of the rolling mill at around 1600 degrees F. j to car e anng S 66 W respon o and has merely air cooled thereafter. Air cooling causes strain precwltatlon hardefnmg the steps fmmpnsmg scale to form on the surface of the stock. The scale Provldmg Sald Stock f a Cross Pectlonal area must be removed, generally by acid pickling or shot- 70 greater than a final, dFSITEd Cross $et10nalarea, blasting, before the stock is drawn because the surface reducing the cross Sectlonal area y pP y a P11re of the stock can be deformed or the drawings dies will tensile stress to the entire cross sectional area of be worn before their economic life has run. In my method the stock until a permanent set 1s imparted to 1t, the stressing of the stock causes the scale to actually pop subjecting the stressed stock to a temperature below t t e e d f the stressing Operation the stock is as the recrystallization temperature of the stock in 1ts unstressed condition to yield a structure having increased physical properties, and

cold sizing the stock to the final desired size,

2. The method of claim 1 further characterized in that the temperature to which the stressed stock is subjected is in the range of from about 200 F. to about 1000 F.

3. The method of claim 2 further characterized in that the temperature to which the stressed stock is subjected is in the range of from about 400 F. to about 600 F.

4. The method of claim 1 further characterized in that the stressed stock is subjected to the aforesaid temperature until the strain or precipitation hardening becomes effective.

5. The method of claim 1 further characterized in that the increase in length imparted to the stock is in the range of from about 2% to about 8% of the original length of the stock.

6. The method of claim 5 further characterized in that the stock is stressed about 6%.

7. The method of claim 1 further including the steps of cold drawing the stock until a change in cross section configuration is achieved subsequent to subjection of the stock to the aforesaid temperature, and

10 again subjecting the stock to a temperature below the recrystallization temperature of the stock in its unstressed condition, said preceding two steps being carried out immedi- 5 ately prior to the final cold sizing operation.

References Cited by the Examiner UNITED STATES PATENTS 1,373,589 4/1921 Blood 14812.9 10 1,692,521 11/1928 Sturcke 14s 12 1,978,219 10/1934 Otte 148-12 2,235,026 3/1941 Kruse 14812.9 2,813,049 11/1957 MacCutcheon 148--12 15 2,848,775 8/ 1958 Ettenreich 148-129 2,924,543 2/1960 Nachtman 14812 3,053,703 9/1962 Breyer 148-12 FOREIGN PATENTS 295,390 1/1929 Great Britain. 373,453 5/ 1932 Great Britain. 400,395 10/ 1933 Great Britain.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, Examiner. 

1. IN THE METHOD OF IMPARTING INCREASED PHYSICAL PROPERTIES TO CARBON BEARING STEEL STOCK WHICH RESPONDS TO STRAIN OR PRECIPITATION HARDENING, THESTEPS COMPRISING PROVIDING SAID STOCK HAVING A CROSS SECTIONAL AREA GREATER THAN A FINAL, DESIRED CROSS SECTIONAL AREA, REDUCING THE CROSS SECTIONAL AREA BY APPLYING A PURE TENSILE STRESS TO THE ENTIRE CROSS SECTIONAL AREA OF THE STOCK UNTIL A PERMANENT SET IS IMPARTED TO IT, SUBJECTING THE STRESSED STOCK TO A TEMPERATURE BELOW THE RECRYSTALLIZATION TEMPERATURE OF THE STOCK IN ITS UNSTRESSED CONDITION TO YIELD A STRUCTURE HAVING INCREASED PHYSICAL PROPERTIES, AND COLD SIZING THE STOCK TO THE FINAL DESIRED SIZE. 